In recent years, as a power generation system which is mild to the global environment and clean, fuel cells have drawn attention. Fuel cells are classified, depending on kinds of electrolytes, into a phosphoric acid type, a molten carbonate type, a solid oxide type, a polymer electrolyte type, etc. Among them, polymer electrolyte fuel cells are tried to be applied as power sources for electric vehicles, power sources for portable apparatuses, and, further, applied to domestic cogeneration systems utilizing electricity and heat at the same time, from the viewpoints of workability at low temperatures, miniaturization and lightening, etc.
A polymer electrolyte fuel cell is generally composed as follows. First, on both sides of a polymer electrolyte membrane having ion conductivity (the ion is usually proton), catalyst layers comprising a platinum group metal catalyst supported on carbon powder and an ion-conductive binder comprising a polymer electrolyte are formed, respectively. On the outsides of the catalyst layers, gas diffusion layers as porous materials through which fuel gas and oxidant gas can pass are formed, respectively. As the gas diffusion layers, carbon paper, carbon cloth, etc. are used. An integrated combination of the catalyst layer and the gas diffusion layer is called a gas diffusion electrode, and a structure wherein a pair of gas diffusion electrodes are bonded to the electrolyte membrane so that the catalyst layers can face to the electrolyte membrane, respectively, is called a membrane-electrode assembly (MEA). On both sides of the membrane-electrode assembly, separators having electric conductivity and gastightness are placed. Gas paths supplying the fuel gas or oxidant gas (e.g., air) onto the electrode surfaces are formed, respectively, at the contact parts of the membrane-electrode assembly and the separators or inside the separators. Power generation is started by supplying a fuel gas such as hydrogen or methanol to one electrode (fuel electrode) and an oxidant gas containing oxygen such as air to the other electrode (oxygen electrode). Namely, the fuel gas is ionized at the fuel electrode to form protons and electrons, the protons pass through the electrolyte membrane and transferred to the oxygen electrode, the electrons are transferred via an external circuit formed by connecting both electrodes into the oxygen electrode, and they react with the oxidant gas to form water. Thus, the chemical energy of the fuel gas is directly converted into electric energy which can be taken out.
Further, in addition to such proton exchange-type fuel cells, anion exchange-type fuel cells using an anion-conductive membrane and an anion-conducting binder (the anions are usually hydroxide ions) are also studied. The constitution of a polymer electrolyte fuel cell in this case is basically the same as in the proton exchange-type fuel cell except that an anion-conductive membrane and an anion-conductive binder are used in place of the proton-conducting membrane and the proton-conducting binder, respectively. As to the mechanism of generation of electric energy, oxygen, water and electrons react at the oxygen electrode to form hydroxide ions, the hydroxide ions pass through the anion-conductive membrane and react with hydrogen at the fuel electrode to form water and electrons, and the electrons are transferred via an external circuit formed by connecting both electrodes into the oxygen electrode and react again with oxygen and water to form hydroxide ions. Thus, the chemical energy of the fuel gas is directly converted into electric energy which can be taken out.
The above electrode reactions take place at three-phase interfaces formed by a gaseous phase as a supplying path of the fuel gas or oxidant gas, a liquid phase as an ion path and a solid phase as an electron path. The ion-conductive binder is used for the purpose of binding the catalyst and heightening the utilization efficiency of the catalyst by mediating the transfer of protons or hydroxide ions from the catalyst layer to the electrolyte membrane. Therefore, catalyst particles not contacting with the ion path formed by the ion-conductive binder cannot take part in the formation of the three-phase boundary, and it is hard for such particles to contribute to the reaction. Further, in order to obtain high efficiency, the minute structural design of the catalyst layer including pore structure for diffusing fuel gas or oxidant gas, the dispersion state of the catalyst, etc. becomes important. Further, at the gas diffusion electrode parts, there arises a case wherein the catalyst surface is covered with water contained in the reaction gases or water formed at the oxygen electrode or the fuel electrode, and the fuel gas or the oxidant gas cannot contact with the catalyst surface, and as a result, power generation is stopped, or a case wherein such water prevents the fuel gas or oxidant gas from being supplied or discharged to stop the electrode reaction. Therefore, the water repellency of the gas diffusion electrode part is required.
As to bonding of the gas diffusion electrode and the electrolyte membrane, a method wherein the bonding is carried out by hot press is known. Further, a method is known wherein, in order to obtain good bonding strength and good electric bonding state, an ion-conductive binder is applied as an adhesive resin between the gas diffusion electrode and the electrolyte membrane to enhance adhesion. In such application, the ion-conductive binder is used generally in a solution sate.
In order to form three-phase interfaces acting as electrode reaction sites, in proton-exchange-type fuel cells, Nafion (registered trademark of Dupont Co., which is the same hereinafter), which is a perfluorocarbonsulfonic acid polymer as described in Patent Document 1 and Patent Document 2, is generally used from the reason of being chemically stable, as the ion-conducting binder (cation-conducting binder). However, Nafion has a hydrophobic main chain having no sulfonic acid group, and it is supposed that the catalyst covered with the hydrophobic part in the catalyst layer cannot form a three-phase interface and cannot contribute to electrode reaction. If the content of sulfonic acid groups is increased for increasing the number of the three-phase interface and increasing catalyst utilization proportion, there is an apprehension that the hydrophilicity of the polymer is heightened, the binder itself is gradually eluted out of the cell system by moisture contained in reaction gases, water formed in the oxygen electrode, etc. during power generation, and deterioration of the electrodes advances to make ensuring of sufficient operation time impossible. If the Nafion content is increased for increasing the content of sulfonic acid groups in the catalyst layer, there is an apprehension that Nafion blocks the diffusion paths of reactants necessary for formation of the three-phase interfaces to lower the efficiency of electrode reaction.
In order to increase three-phase interfaces, an electrode consisting of electrically conductive particles supporting a catalyst thereon and electrolyte particles having proton conductivity is proposed (Patent Document 3). Such an electrode is advantageous for forming diffusion paths of reactant gases, but since the particle size of the resulting particles is as very large as 3 to 20 μm, it is difficult to design minute electrode catalyst layers for increasing three-phase interfaces. Furthermore, since the matrix polymer is used as the electrolyte particles, it is impossible to heighten the ionic group density of electrolyte particle surfaces and it is difficult to form ion paths necessary for formation of three-phase interfaces.
As materials for ion-conductive binders, besides fluorine-containing electrolytes, aromatic engineering plastic resins in which ion-conductive groups such as sulfonic acid groups are introduced is studied. Besides aromatic engineering plastic resins, cation-conductive binders comprising styrene type thermoplastic elastomers have been proposed (Patent Document 4 and Patent Document 5). For example, sulfonated SEBS (SEBS is an abbreviation of polystyrene-poly(ethylene-butylene)-polystyrene tri-block copolymer) is proposed, and it is disclosed that it is sparingly soluble in water, and has good bonding strength with an electrolyte membrane. However, ion-conductive binders comprising the above-mentioned aromatic engineering plastic resin or styrene type thermoplastic elastomer also have the same problems as fluorine-containing ion-conductive binders.
Further, resins are studied which have a high ionic group content and are made to be insoluble in water by introducing cross-linking groups, but the resins are insoluble in organic solvents and need to be used as a suspension in a suitable solvent, and moreover, are poor in dispersibility into the catalyst layer and thus it is difficult to form effective three-phase interfaces.
On the other hand, as anion-conductive binders used in anion exchange-type fuel cells, one wherein the sulfonic acid groups of perfluorosulfonic acid-type polymers are converted to anion exchange groups is known. For example, one obtained by copolymerizing tetrafluoroethylene with CF2═CFOCF2CF(CF3)O(CF2)2SO3F and then making N,N,N′-trimethylethylenediamine act thereon is disclosed in Patent Document 6. As another anion-conductive binder, an anion exchange resin obtained by chroromethylating a copolymer of an aromatic polyethersulfone with an aromatic polythioethersulfone and then aminating the resulting compound is disclosed in Patent Document 7. As still another anion-conductive binder, an anion-conductive binder obtained by introducing a quaternary ammonium group into a polystyrene-poly(ethylene-butylene)-polystyrene tri-block copolymer or the like is disclosed in Patent Document 8. However, the above-mentioned anion-conductive binders also have the same defects as the cation-conductive binders have.
As mentioned above, it is the actual situation that a catalyst layer which is excellent in formation of three-phase interfaces acting as electrode reaction sites, and, at the same time, does not block diffusion paths of reactants.    Patent Document 1: JP 2-7398 B    Patent Document 2: JP 3-208260 A    Patent Document 3: JP 2003-123771 A    Patent Document 4: JP 2002-164055 A    Patent Document 5: JP 2000-513484 A    Patent Document 6: JP 2000-331693 A    Patent Document 7: JP 11-273695 A    Patent Document 8: JP 2002-367626 A